The invention relates to a memory cell configuration with memory elements having a layer structure with a magnetoresistive effect.
Layer structures with a magnetoresistive effect are known from the technology analysis titled xe2x80x9cXMR-Technologien, Technologiefrxc3xcherkennung [XMR Ttechnologies, Technology Detection At An Early Stage]xe2x80x9d, author Stefan Mengel, published by VDI Technologiezentrum Physikalische Technologien. Depending on the construction of the layer structure, a distinction is made between a giant magnetoresistance (GMR) element, a tunneling magnetoresistance (TMR) element, an anisotropic magnetoresistance (AMR) element and a colossal magnetoresistance (CMR) element.
The term GMR element is used by experts for layer structures that have at least two ferromagnetic layers and a nonmagnetic, conductive layer disposed in between and exhibit the so-called giant magnetoresistance effect, that is to say a large magnetoresistive effect in comparison with the anisotropic magnetoresistance effect. The GMR effect encompasses the fact that the electrical resistance of the GMR element is dependent on whether the magnetizations in the two ferromagnetic layers are oriented in a parallel or antiparallel manner.
Tunneling magnetoresistance layer structures have at least two ferromagnetic layers and an insulating, nonmagnetic layer disposed in between. In this case, the insulating layer is so thin that a tunneling current occurs between the two ferromagnetic layers. The layer structures likewise exhibit a magnetoresistive effect that is caused by a spin-polarized tunneling current through the insulating, nonmagnetic layer disposed between the two ferromagnetic layers. In this case, too, the electrical resistance of the TMR element is dependent on whether the magnetizations in the two ferromagnetic layers are oriented in a parallel or antiparallel manner.
The AMR effect is manifested in the fact that the resistance in magnetized conductors is different parallel and perpendicular to the magnetization direction. It is a volume effect and thus occurs in single ferromagnetic layers.
A further magnetoresistance effect, which is called a colossal magnetoresistance effect because of its magnitude (xcex94R/R=100 to 400% at room temperature), requires a high magnetic field for changing over between the magnetization states on account of its high coercive forces.
U.S. Pat. No. 5,640,343 describes a so-called magnetoresistive random access memory (MRAM) cell configuration in which memory cells are disposed between two layers of metallic lines disposed one above the other. The memory cells each have a diode and a memory element connected in series therewith. The memory element has a layer structure with a magnetoresistive effect. The diode is a pn diode or a Schottky diode which contains silicon. The layer structure of the memory element and the layer structure of the diode are disposed above one another. The metallic lines of the first layer run parallel to one another. The metallic lines of the second layer run parallel to one another and perpendicularly to the metallic lines of the first layer. The memory cells are in each case connected between a metallic line of the first layer and a metallic line of the second layer. The layer structure of the memory element contains two ferromagnetic layers and an insulating layer disposed in between. The electrical resistance of the memory element depends on whether the magnetization directions of the two ferromagnetic layers are parallel or antiparallel to one another. In order to write an information item to a memory cell, currents are impressed on the metallic lines that are connected to the memory cell. In this case, voltages are chosen such that no current flows through the memory cell. The magnetic fields generated by the currents accumulate in the region of the memory cell in such a way that the magnetization of one of the two magnetic layers is oriented in the magnetic field. The magnetization direction of the other ferromagnetic layer remains unchanged. The orientation represents the information item. In order to read out the information item, the voltage of the metallic line that is connected to the diode is lowered and the voltage on the metallic line that is connected to the memory element is increased. The same voltage is present on metallic lines that are connected to the remaining memory elements as on the metallic line which is connected to the diode of the memory cell to be read. The same voltage is present on the metallic lines that are connected to the remaining diodes as on the metallic line which is connected to the memory element of the memory cell to be read. On account of the diodes in the memory cells, current can only flow through the memory cell to be read. The current has two discrete values depending on the information stored on the memory cell, which values correspond to two magnetization states of the memory element.
It is accordingly an object of the invention to provide a memory cell configuration and a method for fabricating it which overcomes the above-mentioned disadvantages of the prior art devices and methods of this general type, which can be fabricated with an increased packing density in comparison with the prior art.
With the foregoing and other objects in view there is provided, in accordance with the invention, a memory cell configuration. The memory cell configuration contains at least three layers of metallic lines, and two layers of memory cells disposed in conjunction with the metallic lines alternately one above another. The memory cells each have a diode and a memory element connected in series with the diode. The memory element has a layer structure with a magnetoresistive effect. The diode has a layer structure containing at least two metal layers and an insulating layer disposed in between the two metal layers. The layer structure of the memory element and the layer structure of the diode are disposed above one another. The metallic lines in each of the three layers run parallel to one another. The metallic lines of mutually adjacent ones of the three layers run transversely with respect to one another, and the memory cells are in each case connected between one of the metallic lines of one of the three layers and one of the metallic lines of an adjacent one of the three layers.
The problem is furthermore solved by a method for fabricating a memory cell configuration, in which a first layer of metallic lines that run parallel to one another is produced. A first layer of memory cells is produced above the first layer of metallic lines in such a way that the memory cells of the first layer are connected to the metallic lines of the first layer. A memory element and a diode connected in series therewith are in each case produced for the memory cells of the first layer. For the diode, a layer structure is produced which contains at least two metal layers and an insulating layer disposed in between. A layer structure with a magnetoresistive effect is produced for the memory element. The layer structure of the memory element and the layer structure of the diode are produced above one another. A second layer of metallic lines, which run parallel to one another and transversely with respect to the metallic lines of the first layer, is produced above the first layer of memory cells in such a way that the memory cells of the first layer are in each case connected between a metallic line of the first layer and a metallic line of the second layer. Memory cells of a second layer that are constructed in accordance with the memory cells of the first layer are produced above the second layer of metallic lines. A third layer of metallic lines, which run parallel to one another and transversely with respect to the metallic lines of the second layer, is produced above the second layer of memory cells in such a way that the memory cells of the second layer are in each case connected between a metallic line of the second layer and a metallic line of the third layer.
Since the memory cells are disposed in at least two layers stacked one above the other, the area requirement per memory to element decreases and the packing density of the memory cell configuration is increased. The greater the number of layers of memory cells which are stacked one above the other, the higher the packing density which can be achieved. In this case, each layer of memory cells is disposed between two layers of metallic lines. The metallic lines of one layer run transversely with respect to the metallic lines of the other layer. The metallic lines can each be fabricated with minimum dimensions and spacings of a minimum feature size F which can be fabricated in the technology used, resulting in an area requirement per memory cell of 4F2 per layer. Overall, an area requirement of 4F2/n per memory cell is produced in the memory cell configuration given n layers.
The diode is a tunnel diode in which electrons tunnel through the insulating layer more easily in one direction than in the other. High temperatures that could destroy metallic lines situated under the diode are not necessary for producing the diode.
The provision of the diodes prevents the situation where, during the read-out of information of a memory cell of a layer, currents flow through memory cells of a layer adjacent to this layer. Furthermore, the provision of the diodes has the effect that a current flows exclusively through the memory cell to be read between the metallic lines that are connected to the memory cell to be read. This is advantageous since the magnitude of the current is thereby independent of information items stored on other memory cells and only represents the information item to be read out.
The process outlay for producing the memory cell configuration is particularly low if a mask for producing the metallic lines of one layer and a mask for producing the metallic lines of an adjacent layer simultaneously serve for producing the memory cells disposed between these layers of metallic lines.
By way of example, a first conductive layer and, above the latter, layers for producing the first layer of memory cells are produced. The layers for producing the first layer of memory cells and the first conductive layer are patterned with the aid of a strip-type first mask in such a way that the first layer of metallic lines is produced from the first layer. Afterward, an insulating material is deposited and planarized, thereby producing mutually separate strip-type first insulating structures.
A second conductive layer and layers for producing the second layer of memory cells are applied. The layers for producing the second layer of memory cells, the second conductive layer and the layers for producing the first layer of memory cells are patterned with the aid of a strip-type second mask in such a way that the second layer of metallic lines is produced from the second layer, and that the memory cells of the first layer are produced from the layers for producing the first layer of memory cells. The memory cells of the first layer are consequently produced in two steps by patterning the corresponding layers. The first step takes place in the production of the first layer of metallic lines and the second step takes place in the production of the second layer of metallic lines. Afterward, an insulating material is deposited and planarized, thereby producing mutually separate strip-type second insulating structures.
Afterward, a third conductive layer is applied. The third conductive layer and the layers for producing the second layer of memory cells are patterned in a strip-type manner in such a way that the third layer of metallic lines is produced from the third layer, and that the memory cells of the second layer are produced from the layers for producing the second layer of memory cells. The memory cells of the second layer are consequently produced by patterning the corresponding layers in two process steps. The first step takes place in the production of the second layer of metallic lines and the second step takes place in the production of the third layer of metallic lines. In this way, it is possible to produce as many layers of memory cells and metallic lines as desired.
Particularly good electrical properties of the diode can be achieved if the difference between the work function of one metal layer and the work function of the other metal layer is as large as possible. In this case, the asymmetry of the tunnel effects in the diode is particularly large, i.e. electrons tunnel significantly more easily in one direction than in the other. Aluminum has a particularly low work function. Consequently, one metal layer is preferably composed of aluminum. However, other materials having a low work function are likewise suitable. The other metal layer is preferably composed of platinum, since platinum has a particularly high work function. However, other materials having a high work function, such as e.g. tungsten, are likewise suitable.
In order to produce the insulating layer, the aluminum can be oxidized.
The metallic lines may be composed, for example, of Cu, AlSiCu or metal silicide.
In order to simplify the process, it is advantageous if one of the two metal layers of the diode is part of one of the metallic lines. In this case, the metallic lines are preferably composed of aluminum.
A diffusion barrier made of TiN, for example, may be disposed between the layer structure of the diode and the layer structure of the memory element.
All known TMR elements and GMR elements are suitable as memory elements. Furthermore, all XMR elements are suitable which have two magnetization states with a different resistance, between which it is possible to switch back and forth by application of a magnetic field whose magnitude is acceptable for the memory application.
Preferably, the layer structure of the memory element at least contains two magnetic layers and a nonmagnetic layer disposed in between. The memory elements each have two magnetization states.
The nonmagnetic layer is preferably an insulating layer (TMR element) because this makes it possible to achieve higher element resistances (=100 kxcexa9), which are more favorable with regard to power consumption and signal/noise ratio. Examples of suitable materials for the magnetic layers are Ni, Fe, Co, Cr, Mn, Gd, Dy and alloys thereof, such as NiFe, NiFeCo, CoFe, CoCrFe, and MuBi, BiFe, CoSm, CoPt, CoMnB, CoFeB. Examples of suitable insulating materials for the nonmagnetic layer are Al2O3, MgO, NiO, HfO2, TiO2, NbO, SiO2 and DLC (diamond-like carbon). Examples of suitable conductive materials for the nonmagnetic layer are Cu or Ag.
The thickness of the magnetic layers is preferably between 5 nm and 10 nm. The thickness of the nonmagnetic layer preferably lies in the range between 1 nm and 3 nm. The memory elements preferably have dimensions in the range between 50 nm and 150 nm. They can have, inter alia, a square or elongate configuration.
In order to increase the process reliability of the method, it is advantageous to dispose an intermediate layer between the layer structure of the memory cell and a metallic line disposed thereon. The intermediate layer prevents damage to the layer structures of the memory cell during the planarization of the insulating material for the purpose of producing the insulating structures. During the planarization, the intermediate layer is uncovered and can also be removed somewhat. Furthermore, the intermediate layer can simultaneously act as a diffusion barrier.
The layer structure of the memory elements may be disposed on the layer structure of the diode. As an alternative, the layer structure of the diode is disposed on the layer structure of the memory element.
One possible method of operation of the memory cell configuration is described below.
In order to write information to a memory cell, currents are impressed on the metallic lines connected to the memory cell. A magnetic field generated as a result is larger in the region of the memory cell than in regions of the remaining memory cells. In the region of the memory cell, the magnetic field is so large that the magnetization of the magnetically softer of the two magnetic layers of the associated memory element is oriented in the magnetic field. The magnetization direction of the magnetically harder of the two magnetic layers remains unchanged. The magnetic field is set in accordance with the information to be written such that the magnetization direction of the magnetically soft layer is parallel or antiparallel to the magnetization direction of the magnetically hard layer. Consequently, the memory cell can assume two different magnetization states.
In order to read out the information of the memory cell, voltages are applied to the associated metallic lines in such a way that a current flows through the memory cell. The magnitude of the current is dependent on the magnetization state of the memory cell, and is consequently dependent on the stored information.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a memory cell configuration and a method for fabricating it, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.